3(b) Flashcards
What is a gene?(2)
A gene is a small section of DNA that determines a particular feature.
A gene can also be described as a section of DNA that codes for a particular protein.
Describe the structural features of DNA?(3)
A nucleotide is the repeat unit within DNA and is made up of a phosphate group, sugar molecule and nitrogenous base(either Adenine, Thymine, Cytosine, or Guanine).
DNA has a double helix structure.
It should also be noted that the two ‘strands’ go antiparallel to each other.
Why is DNA described as a triplet code?(2)
The template strand in DNA codes for amino acids, amino acids chain up to form proteins.
Each amino acid is coded by three bases in the template strand - i.e TGT codes for the amino acid ‘cysteine’.
Describe the stages of protein synthesis?(2)
DNA remains in the nucleus but protein synthesis takes place in the cytoplasm. Therefore, to make proteins, DNA code must be copied and then transferred to the cytoplasm.
Ribonucleic acid(RNA) is used to carry out this.
Describe the process of transcription?(3)
Transcription happens in the nucleus, and starts when in a chromosome, the DNA double helix unzips so that the two strands separate exposing the bases.
The template stand of the DNA forms a frame onto which a mRNA molecule forms according to the complementary base pairing rules.
Once the section of the DNA corresponding to a protein has been transcribed, the mRNA molecule leaves through pores in the nuclear membrane.
Describe the process of translation?(4)
Translation happens at the ribosomes. At this point, the code in the mRNA consists of triplets i.e (AUG) called codons.
Once a mRNA molecule attaches to a ribosome, the tRNA molecules use their anticodons to code for an amino acid.
After this amino acid has been created, the tRNA molecule will then go and collect another amino acid.
The chain of amino acids eventually continues until a triplet codes for ‘stop’ and the protein has been created. The polypeptide chain of amino acids is what is called a protein.
Explain the different types and causes of mutation?(3)
Mutation is a random change in the DNA of a cell. It can happen in individual genes or in whole chromosomes.
There are four main types of mutation within genes;
deletion, duplication, substitution and inversion.
Only if a mutation is within a gamete or in cells that divide to form gametes, then it has the possibility of being passed on.
Describe the structure of chromosomes?(2)
Each chromosome contains a double-stranded DNA molecule. The DNA is folded so it can be packed into a small space.
The proteins that DNA is coiled around are called histones.
What is phenotype and what is genotype?(2)
An individual’s genotype is the combination of alleles that they possess for a specific gene.
An individual’s phenotype is the combination of their observable characteristics or traits.
What is an allele and genome?(2)
The entire DNA of an organism in a diploid cell is known as its genome.
Within DNA there are genes which control the production of proteins in a cell. Within these genes there are alleles which are forms of a gene but only a section.
Define dominant and recessive, homozygous and heterozygous.(2)
A dominant allele is one that only needs one haploid cell within the zygote for it to posses the phenotype meanwhile a recessive allele is one that needs both haploid cells to posses the recessive allele as the phenotype.
Homozygous means two alleles of the same genotype while heterozygous means two alleles with different genotypes.
Describe and explain Mitosis?(5)
Mitosis is the copying of a single cell to produce two genetically identical diploid cells.
Mitosis occurs whenever cells need to be replaced in our bodies, whenever we need growth, asexual reproduction or cloning.
First the DNA replicates and the chromosomes form two exact copies called chromatids.
The chromatids line up in the middle of the spindle which then after the fibres contract, pull the chromatids to the opposite ends of the cell.
On either end of the cell there are now exactly identical chromosomes. Two new nuclei form at the poles of the cell and the cytoplasm divides to form the cell membrane of each new daughter cell.
Describe and explain Meiosis?(4)
During meiosis a cell will copy each chromosome and then divide twice.
Meiosis is done when haploid cells such as sperm or eggs need to be created.
First the chromosomes of a parent cell pair up in homologous pairs.
Then each pair of chromosomes(2) split up into separate cells, these then separate again however without copying, so that in each of the four cells produced, there are only 23 chromosomes.
What is co-dominance?
If two alleles are expressed in the same phenotype, they are called co-dominant. Most genes do not show complete dominance, most variation that is continuous is due to co-dominant alleles.
How is humans sex determined at fertilisation?(2)
A single gene determines whether humans are male or female.
It is determined by the X and Y chromosomes. Both a male and female will always have an X chromosome, but a female will have XX and a male XY.
